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Happy Hour

Jen-Luc Piqunt stumbled across an intriguing science news story this morning: it seems that engineers at Ohio State University "have invented a new kind of nano-particle that shines in different colors to tag molecules in biomedical tests." The secret ingredient? quantum dots! We love quantum dots at the cocktail party, and they rarely make news headlines. This seems like a good time to indulge in a spot of self-plagiasm and adapt some information from my 2007 post on the subject.

Quantum dots are tiny bits of semiconductors -- sometimes called nanocrystals, which just doesn't carry the same panache -- just a few nanometers in diameter. It's like taking a wafer of silicon and cutting it in half over and over again until you have just one tiny piece with about a hundred to a thousand atoms. That's a quantum dot. Billions of them could fit on the head of a pin.

Size matters when it comes to semiconductors: smaller is usually better. Because they're so tiny, quantum dots have some unusual materials properties -- specifically, the all-important electrical and optical ones -- thanks to the quantum effects that kick in at smaller size scales, so they are of enormous interest to researchers. It's interesting physics fundamentally, and it offers an impressive sampling of potentially lucrative practical applications.

It helps to place semiconductors in general in the appropriate context, i.e., right smack between insulators and conductors. Insulator atoms hoard their electrons greedily, like misers or overprotective parents, and rarely part with them, while conductor atoms are like spendthrifts or exceedingly permissive parents, letting their electrons run amok all over the place (and a good thing, too, otherwise we'd never enjoy the benefits of electrical current).

Semiconductor atoms are juuuust riiiight. They don't fling their electrons around all willy-nilly, but neither do they hang onto to them too tightly. It takes a bit of an energy boost to knock an electron loose in a semiconductor, and when the electron breaks free, it leaves behind a "hole" in the atom's electronic structure -- a vacancy, if you will, that another electron, sooner or later, will come along to fill. So a photon strikes a semiconductor atom and creates an electron-hole pair. This enables the electrons to flow as a current. And current = power.

Back in 1990, European researchers managed to get porous silicon to emit red light, and figured it came about because of "quantum confinement" relating to the dot's small size. At 10 nanometers or less, the electrons and holes are being squeezed into such small dimensions that this alters the electronic and optical properties; it's the critical feature of most nanoscale materials, frankly. Things snowballed from there, with scientists making more silicon dots (and, later, germanium dots) that emitted light in lots of bright, pretty colors, especially the highly desirable green and blue ranges. The bigger the dot, the redder the light, and the emitted light becomes shorter and shorter in wavelength -- and higher in energy -- as the dots shrink in size. This is called "tunability" because you can pretty much tailor the dots to emit whatever frequency of visible light you happen to need for a given application, simply by altering the size of the dots

The most obvious application is using quantum dots as an alternative to the organic dyes used to tag reactive agents in fluorescence-based biosensors. You know, the dyes start to glow when, say, a harmful toxin is present. But the number of colors available using organic dyes is limited, and they tend to degrade rapidly. Quantum dots offer a broader spectrum of colors and show very little degradation over time. Having all those colors also means you can make light-emitting diodes (LEDs) from quantum dots, precisely tuned in the blue or green range. You can also build quantum dot LEDs that emit white light for laptop computers or interior lighting in cars. As for electronics, the possibilities are endless: all-optical switches and logic gates, for instance, with a millionfold increase in speed and lower power requirements, or, further in the future, quantum dots could be used to make teensy transistors for nanoelectronics.

This latest breakthrough -- described in the online edition of Nano Letters, in a paper by OSU's Jessica Winter and Gang Ruan -- involves sutffing tiny plastic nanoparticles with even tinier quantum dots for use in biomedical tagging applications. It's easier to see biological molecules under a microscope if they fluoresce, and quantum dots glow more brightly than other fluorescent molecules used for this purpose.

They also "twinkle", i.e., blink on and off, an effect that is less noticeable if there are many quantum dots congregated together. There are pros and cons to this behavior. Con: it "breaks up the trajectory of a moving particle or tagged molecule" that one is trying to track under the microscope. Pro: when the blinking stops, scientists know they've reached a critical threshold of aggregated quantum dots. What Winter and Ruan have done to address this is to turn that "con" into another "pro" by stuffing quantum dots of different colors into the same micelle (a polymer (plastic) based spherical container commonly used in lan experiments). Their tests showed that doing show caused the micells to glow steadily. To wit:

"Those stuffed with only red quantum dots glowed red, and those stuffed with green glowed green. But those he stuffed with red and green dots alternated from red to green to yellow. The color change happens when one or another dot blinks inside the micelle. When a red dot blinks off and the green blinks on, the micelle glows green. When the green blinks off and the red blinks on, the micelle glows red. If both are lit up, the micelle glows yellow. The yellow color is due to our eyes' perception of light. The process is the same as when a red pixel and green pixel appear close together on a television or computer screen: our eyes see yellow."

The continuous glowing makes it easer to track tagged molecules with no breaks, and they can also use the color changes to determine when said tagged molecules congregate. The new nanopartices would be great for microfluidic devices, and could one day be combined with magnetic particles to enhance medical imaging for, say, cancer detection. So it's nice to see quantum dots getting a little love in the public sphere again.

UPS used to run commercials bragging that they kept their planes immaculately clean because a clean plane has less drag and saves energy. They didn't mention how much energy it takes to produce the water and soap, or to treat the water after it's been used. Most of us didn't really care all that much, either. As airplane prices start to rise with the new geopolitical unrest, people are paying more attention and realizing that sometimes, little things can mean a lot.

One of the most difficult basic physics concepts to accept is that no energy is required to keep an object moving at constant speed if there are no forces acting on the object. Force is required to create acceleration, not velocity. You must supply a force to throw a ball in the air, but in the absence of any external forces the ball would go on forever in the same direction once it got started.

The primary reason why this is so difficult for students to accept (and why even very intelligent people like Aristotle got it wrong) is because a person is highly unlikely to encounter a situation in which an object is not subject to friction. Unless you live in space, every object you encounter is going to be affected by gravity, friction and air resistance. This remains one of my primary pet peeves about how we teach physics. Why do we insist on starting out by telling students things that contradict everything they have experienced in life thus far?

Air resistance is the friction created when air molecules bounce off a surface. While the up/down forces are important for keeping the plane in the air, we are also interested in the force the air molecules exert in the direction opposite to the direction in which the plane is moving. The sum of the tiny forces exerted by the very large number of molecules that oppose motion is called drag. Drag is one of those external forces that require the plane to constantly supply energy. The higher the drag, the more fuel required to keep the plane moving at a constant speed.

Drag depends on a lot of things, including the density of the air (lower density means fewer molecules encountered during the same time), the cross-sectional area of the plane (a bigger area hits more molecules) and speed (a faster plane hits more molecules in the same time).

The concept of area is interesting when you're talking about friction because the critical parameter isn't so much the macroscopic area, but some type of surface area that takes into account the surface roughness on a micro- or even nano-scopic sense.

I've discussed this in reference to surfaces, like wood and skin, but it applies to everything: It may look and feel smooth to the eye, but If you look at any surface closely enough, you will find roughness on some scale. As you might imagine, a perfectly smooth surface will produce less drag than a rough surface.

Aerodynamics is very important to racecars when engine power is limited. When going to tracks like Daytona and Talladega, NASCAR teams apply dummy decals to the bare metal of the car's body. They paint the car, and then remove the decals. Keeper decals are applied in the relief cavities made by the paint, and then the car is clear coated and sanded smooth. Even the roughness associated with the raised edge of a decal is large enough to be of concern to a race team. A small difference in drag can have a big impact in results when races are won and lost by thousandths of a second.

Commercial aviation isn't concerned as much about speed as it is with cost. According to portfoilio.com, fuel prices usually account for 30 percent of operating costs; however, when oil prices keep rising (as they are doing now with the increased unrest in northern Africa), they can account for 40 percent of the cost of a flight.

I'm suspicious of 'magic numbers' like the $100/barrel mark, but it makes sense that there exists a tipping point for air travel that probably isn't too removed from the $100/barrel mark. Ticket prices rise with oil prices. At some point, ticket prices are so high that businesses stop approving travel and leisure travelers stay closer to home. The airlines cut prices to attract travelers, and end up flying planes that are actually losing them money. Unless, of course, they find ancillary fees they can tack on for checking bags, drinking soda or reading magazines.

In December 2010, U.S. airlines used 929 million gallons of fuel at a cost of 2.141 billion dollars. A 1% decrease in fuel comsumption represents a savings of 21 million dollars in just one month. When fuel prices rise rapidly, so does industrial interest in technologies that can make even small improvements in efficiency. Dirt sitting on the surface of the plane makes the surface rougher and increases drag; however, washing is not without cost. You have to pay for the person power, the water, the tools and the detergent. Wouldn't it be nice if there were a way to Scotchgard against surface roughness?

That's a little harder said than done. Planes undergo extremes in heat and cold, and if you think the wind on a day with a -20 F wind chill feels like needles on your face, imagine the force a surface must sustain moving at 500 mph at 35,000 feet. Anything applied to the surface of the plane has to be flexible enough to sustain the thermal cycling of the surface as it gets warm and cold, and has to be able to bond to the surface without coming off.

There's an inherent penalty for coatings: American Airlines applies minimal paint to their planes because paint adds mass. In space flight, where the cost per pound is even more significant, not painting the external fuel tanks on the space shuttle saves 600 pounds. An unmanned launch costs more than $10,000 per pound, so even saving a few pounds on a spaceflight represented a major cost advantage. It's not as significant on airplanes, but the coating's benefit has to offset the monetary and weight cost of applying the coating. They can't be too expensive to apply, they have to last long enough that they aren't constantly having to take planes out of the fleet to re-apply the coating, and for commercial airlines, they have to be adaptable to the branding and required identification.

Wax was one of the first vehicle coatings that made the surface smoother and shinier. Wax is applied to the car and forced into the pores and scratches by physically pushing it in, then removing any extra wax. It's a lot of work, but the thin coating that results from a well-applied waxing repels water, increases shine, and smooths the surface. Teflon or silicone coatings also have been used - those are normally small particles of the filler that are suspended in a liquid. The liquid flows onto the surface, the particles are supposed to fill in the indentations, and then the liquid solidifies. The smoothness of the surface, however, is going to depend on the size of the particles and how effectively you can get them into the nooks and crannies. As you can see at left, the smaller the particle, the smoother the final surface. Nanoparticles can get into smaller indentations and there is a lot of effort in developing drag-reducing nanomaterials for everything from yachts to the blades of wind turbines.

EasyJet, a UK aviation company, is testing out a new nanoscale coating for their planes that promises to effectively reduce drag by about 40%. That reduction is expected to translate to about a 2% savings in fuel consumption. Although that doesn't seem like much, EasyJet estimates it would save them about 22 million dollars from their annual fuel bill of about 1.2 billion.

The coating is called TripleO protective system, which has the acronym "ooops". This was not unfortunate, it was intentional, as the princiapl from the company that developed this system owns a chain of auto repair stores. The material being tested on the British jets uses an unspecified nanotechnology that the company says crosslinks with the paint to form a durable coating. At less than a tenth the diameter of a human hair thick, the additional mass of the coating is negligible.

One of the problems with coatings is getting them down into the nooks and crannies so that they form a really strong bond with the surface they are coating. Delamination or spalling is the term used when a film separates from the surface it is covering. It's what happens when your nail polish flakes off. TripleO overcomes the problem of how to get the material down into the cracks by washing the plane first with oxalic acid. The materials on the company's website note that this creates a positive charge on the surface, so I'm guessing that the acid dissolves a very thin layer of paint, leaving a bunch of atoms desperately looking for something with extra electrons that can offset the positive charge. The company's polymer-based coating is negatively charged, so the surface actually pulls the emulsion into the crevices. The coating bonds with the paint to form a smoother surface. As a bonus, the surface also repels dirt, which reduces how often you have to wash the plane.

EasyJet is running an experiment. They've coated eight planes and will compare the fuel mileage of those planes with the other 192 planes in their fleet. If the fuel savings are significant relative to the cost of the application and maintenence of the coating, they'll coat the remaining planes. If EasyJet saves save 2% of their fuel costs, that would correspond to 40 million dollars a month if the mainstream U.S. Aviation fleet followed suit. Possibly more important than how much money the company saves on fuel is that the planes are burning less fuel and thus generating fewer emissions. It seems like a win-win situation.

TripleO has worked with the auto industry, aviation (including British military planes) and even yacht racing, where drag is a major issue in speed. If I ran TripleO, I would ship a couple quarts of the product to Charlotte in time enough to get it on a stock car at Talladega in April. If their claim of decreasing drag by almost 40% is true, I can't see how it isn't a perfect solution: it's lightweight, clear, compatible with paint and it's not illegal. Yet.

Quick: what's the difference between an 'amu' (atomic mass unit) and a 'Da' (Dalton)? Answer: Nothing. They both represent one-twelfth of the rest mass of an unbound carbon-12 atom in its nuclear and electronic ground state, a.k.a 1.66×10−27 kg. This is very slightly less than the mass of a proton or a neutron (approximately 1.67x10-27 kg). When first invented, the Dalton was intended to be a fundamental unit such that one hydrogen atom had a mass of one Dalton. Helium would be two Daltons, lithium would be three Daltons, etc. Of course, then we realized that every atom had different numbers of protons, neutrons and electrons, which mean that there was no simple universal mass. It would be so much easier to memorize if everything on the periodic table was a simple multiple of a fundamental quantity.

Happily, the universe is not that simple. Protons, neutrons and electrons make things just a little more complex. So regardless of whether you prefer the amu or the Dalton, neither is actually fundamental.

I had to look up the difference after attending a seminar last Friday by Kris Noel Dahl from our Neighbors to the North, CMU. Her topic was the interaction of single walled carbon nanotubes (SWCNTs) with cells. The extremely high strength of carbon nanotubes make them ideal for applications such as high performance racing bike frames, tennis racquets, and space elevators - to name just a few.

Nanomaterials surprised the materials and biological sciences communities in multiple ways. Yes, nanoscale materials have amazing properties that the exact same materials in bulk can only dream about; however, they also have different types and degrees of toxicity. A material that is harmless in a centimeter-sized chunk becomes a killer when shrunks to nanoscale.

Carbon nanomaterials, especially, have engendered a lot of concern, with early high-profile reports of buckyballs being toxic to fish brains, for example. There was a lot of backstepping when people realized that as-synthesized carbon materials have a wide range of materials, including graphene, graphite, metal impurities from the catalysts used in some methods to grow the carbon nanomaterials, and even contamination from residual solvents that were used for dispersing the nanotubes in a fluid. Even though we have better methods for purifying carbon nanotubes and removing impurities, there reamin a wide variety of opinions on the toxicity of carbon nanotubes. Most of the research has moved from the 'is it or isn't it toxic' type to 'what specifically do carbon nanotubes change in a cell?' I learned one consequence on Friday that involves an interesting molecules called actin.

Actin is ubiquitous: If you're looking for a molecule that is fundamental to life, this is one to consider. It's a 42-kiloDalton (meaning big) globular protein that varies in structure by less than 20% across species from algae to people. Actin is found in all eukaryotic cells, which are the types of cells that have a nucleus. Eukaryotic literally means 'good nut' or 'good kernel', so defining is the presence of the nucleus.

The globular protein (called G-actin) is a monomer, which means that it assembles with similar momomers to form long-chain polymers. Thin filament-actin is mostly found in muscle cells, forming a scaffold on which myosin motors move - the mechanism by which the muscles contract.

Microfilament actin (also called f-actin) is a major component of cellular cytoskeletons. Two long-chain polymers twist together, like two-ply yarn, to form f-actin (shown to the left). The result is a helix about 7 nm in diameter, with a repeat distance for the twist of about 37 nanometers.

Confession time. My model of the cell is way dated. The model I still had in my head was from the last biology course I ever took: the required 9th grade general biology. We had filled a plastic bag (the cell membrane) halfway with jello (the cytoplasm), let it set awhile, dropped in a maraschino cherry (to represent the nucleus), and then filled it up with more jello.

I knew cells were slightly more complicated than that, but I didn't appreciate how much. On an educational note, jello cell models have also increased in complexity. The picture at right is from a homeschoolers blog. Sugar-coated gummy worms represent the rough endoplasmic reticulum, while smooth gummy worms represent the smooth endoplasmic reticulum (which folks in the know call the 'ER'). Gumdrop centrosomes, Sixlet lysosomes, raisin mitrochondria, Gobstopper vacuoles and sprinkle ribosomes complete the cell. Oops - I almost left out the fruit roll-ups folded accordian-style to represent the Golgi bodies.

I'm getting a sugar buzz just describing this rather colorful model that looks to me way too much like an eyeball to even think about eating it. Despite it's color and ability to keep kids busy, this model -- like almost all models -- has a flaw: You have to make your cell in a mold. Mother Nature doesn't need a mold. And cytoplasm isn't really quite as structurally sound as gelatin, but Mom Nature has a secret ingredient: actin. Actin provides a cell's skeleton. Actin is why red blood cells are flat and even why cells move.

The micrograph of the rat kidney cells below shows the actin cytoskeleton in green and the nuclei in blue. The image was taken by Christopher Turner's group at Upstate Medical University of New York using fluorescence microscopy. The actin filaments adhere to the membrane and provide structural support, but also provide the hard-wiring for cell functionality.

The cellular cytoskeleton is not permanent like our skeletons become: actin can polymerize and depolymerize, changing from long strands to shorter strands or even back to the original globular form. This joining and dissolving can even be used by the cell to move like a snail. The actin cytoskeleton provides mobility and preserves shape. If you change the actin cytoskeleton, you change not only the shape and structure of the cell, you can change the cell's function as well.

Since actin defines a cell's shape, you might infer that actin plays a very important role in cell division - and you'd be correct. The actin forms smaller fibers and distributes itself around the cytoplasm prior to and during cytokinesis (dividing). In the picture at left of dividing green urchin zygote cells (from the University of Washington Center for Cell Dynamics), the actin is in blue and the gold threads are microtubules. Cell division is the basis of life, of course, since it is how we (and most everything else on Earth) reproduce.

Changing the actin structure thus challenges the cell's ability to maintain it's shape, divide, and even to function, which returns us to the subject of the original seminar.

Dahl and her co-workers studied highly purified carbon nanotubes that had been length-selected to be 150 nm long, which is about the length of the f-actin in the cells they were studying. A cell, by contrast, is tens of microns in diameter. Normally, f-actin in HeLa cells concentrates in the cell's base. Dahl's group found that introducing carbon nanotubes changes the way actin organizes. Outside the cells, they found that carbon nanotubes make actin fibers bunch up into bundles like twigs tied up in a bunch. When they looked at the effect of the carbon nanotubes inside cells, the actin again formed clumps, but there was also more actin and the clumps weren't located only in the base of the cell - the clumps were distributed throughout the interior of the cell. The carbon nanotubes also impacted the ability of the cells to divide, producing defects like cells with multiple nuclei and cells that started the dividing process, but couldn't complete it.

This study reinforces a very important issue regarding toxicity. We sometimes think of toxicity as being when something causes cells to die in large numbers. In this case, the carbon nanotubes didn't kill large numbers of cells directly -- but they did hinder the cells from dividing. If we could target carbon nanotubes so that they only entered cancer cells, for example, we would have a technique to slow or stop the growth of cancer. Even slowing cancer cell growth would give us more time to treat it. Carbon nanotubes exposed to a dividing embryo would be bad.

The more I learn, the more I realize that toxicity is a much more subtle phenomenon than I initially appreciated. It's vitally important for us to understand those subtleties so that we can determine not whether nanomaterials are dangerous, but the conditions under which nanomaterials -- or any materials -- could be hazardous. The first step to preventing a potential hazard is to understand it.

Some words are just so much fun to say. My father claimed that, as a child, I was inordinately amused by long Latin words from his legal texts. I would giggle at jurisprudential terms the same way most kids got excited by toys. Or so he always said.

No wonder I turned out the way I did.

But you must admit that viscoelasticity is just a cool word. Like most words, you can break it down into its component parts: viscous and elastic. And like the most interesting thing in physics, it's almost always "beyond the scope of the course". You can talk in a limited way about elasticity (spring constants) and sometimes you'll learn a little about viscosity, but viscoelasticity consistently fails to make it into most introductory physics books. That's too bad because it's not only a fun concept, it is a useful concept.

My first introduction to applied viscoelasticity was my foray into understanding the physics of tires. Rubber is one of the most fascinating materials in the world. Not coincidentally, rubber is viscoelastic. Remember the coefficient of friction? The force it takes to start something sliding is proportional to how much force is pushing down on it. The proportionality coefficient is the coefficient of (static) friction. A coefficient of friction of 0.3 means that a 100-pound block must be pulled with 30 lbs of force before it can start to move. Most classes normally deal with simple cases of one solid material (like wood) on another (like sandpaper). Something like rubber on asphalt might get up to a coefficient of friction of 0.7-0.8.

Before NASCAR, I didn't realize you could have a coefficient of friction greater than one. A coefficient of friction greater than one means that if you want to start something sliding, you have to pull or push the object with a force greater than its weight. In normal-person talk, that means the object is sticky, or changing shape, or any one of a number of interesting things that real objects do when pushed or pulled.

A good race tire can have a coefficient of friction anywhere from 1.2-1.5. That's what it literally means to 'grab' the road. A race tire doesn't feel sticky to the touch, but get it heated up a bit and it feels a really strong affinity for the track, which means you can go fast around the corner without sliding. Viscoelasticity at work.

The linear relationship between the amount of force pushing down and the force needed to slide only holds out so long. Yep, ol' Ff=μN only works within certain limits. That's OK - Physics is so much more interesting when you push its buttons.

Viscous means able to flow under shear (or tensile) stress. Shear means that you push on an object somewhere and one part of the object moves relative to other parts of the object, as I've tried to draw at left. Molten glass, for example, will flow when you pull it. Honey is the canonical viscous material. The more viscous a material is, the more difficult it is for it to flow. And, of course, viscosity is highly dependent on temperature. Warm syrup is much less viscous than cold syrup, which is why it is easier to pour.

Viscosity relies in large part on the molecules in the solid or liquid being able to move relative to each other. If the molecules or atoms were locked in place, you'd have a solid and it wouldn't change shape readily.

The problem with a material like honey is that the ease with which molecules slide past each other is just a little overdone, which makes honey somewhat messy, even if it is highly yummy. If your honey bear tips over at the table, you are highly unlikely to see the honey pull itself back into the bottle. Although wonderfully viscous, there aren't enough molecular interactions to make honey elastic.

Although viscosity relies on the molecules being able to move relative to each other, elasticity requires that those molecules exert some type of restoring force on each other - a type of molecular peer pressure that tells the moving molecules, "hey - get back here!"

Elastic implies the ability of an object to return to it's original shape, or at least a shape something like the original shape. A stretched rubberband deforms, and then returns to its original shape when you remove the deforming force. The combination of viscosity and elasticity however, an object that is viscoelastic has time dependence. If you get one of those big heavy rubberbands and hang something heavy from it, it will stretch. Over time -- and time is the important parameter in viscoelasticity -- the molecules in the rubber band will shift their positions to adapt to the stress of the weight pulling down. It's a little like you or me shifting our position when we're standing for a long time. When the weight is removed, the rubber band molecules can return to their original configuration (or something close to it).

There are a lot of different types of viscoelastic materials. Semi-crystalline and amorphous polymers, both made of poorly ordered (or totally disordered) long chains of molecules tend to be viscoelastic. If you heat a metal to high enough temprature, it will be viscoelastic as well. Biological structures like muscles and blood vessels are viscoelastic.

Polymeric and biological viscoelastic materials are mostly composed of long chains of intertwined molecules. The importance of intertwining was demonstrated by the discovery of vulcanization, which does not mean the conversion of someone into a Star Trek fan.

The sap of many trees (like the rubber tree) is naturally much more visco than elastic, which means it is more like chewing gum than tire rubber. Tree rubber originally was used for erasers - like those gummy art erasers that fall apart and make a mess everywhere. Natural rubber was pretty much useless for most anything else. It got stinky and soft in hot weather, while cold weather made it brittle and cracked. Early Mayan people knew how to vulcanize rubber, but that information was somehow lost between then and 1830s New England.

The frequently bankrupt businessman Charles Goodyear was obsessed with finding a way to make rubber stronger and less sticky without losing its elasticity. The story has it that, as Goodyear was showing off his latest result to a snickering group at the Woburn, Massachusetts general store in 1839, the piece of rubber he had been gesturing with flew into the air and landed on a stove.

Goodyear expected to be cleaning up a gooey mess; however, the material on the stove wasn’t melted. It was elastic, durable and–unlike his previous attempts–it didn’t stick to everything. A true Eureka moment. Goodyear experimented some more and eventually settled on using steam to cure the sulfur-doped rubber. The curing process was called vulcanization after Vulcan, the Roman god of fire and volcanoes. Goodyear, the poor guy, never profited from his discovery. (Bonus trivia: Goodyear never worked for the Goodyear Rubber and Tire Company.)

Vulcanization is the key - you have to get those chains interconnected to each other, otherwise there's nothing inside the material that pushes back when you push on it.

If you take a handful of uncooked spaghetti and push anywhere, those pieces of spaghetti move. Now cook the spaghetti. It becomes a tangled mess and it is much harder to move one strand without the others pulling on it, trying to keep it from moving (or moving along with it).

Using this principle of crosslinking, you can tune a fairly large set of parameters to make a polymer material as hard, soft or elastic as you want it to be. The challenge is that most materials have these desirable properties over fairly limited temperature ranges. A liquid-nitrogen cooled racquet ball shatters instead of bouncing. Tires lose grip when they start to get warm because the heat allows the viscous nature of the rubber to win out over the elastic nature of the rubber.

The molecular motion that controls the balance between viscous and elastic in polymers is thermally activated. That means the rate of motion depends the exponential of the ratio of an energy to kT. (k being the Boltzmann factor, T being the temperature). This exponential dependence means that the motion is extremely sensitive to temperature (much more so than a power law, for example) and thus the ideal viscoelastic properties only last over a limited temperature range.

In the December 3rd issue of Science magazine, a group from AIST in Tsukuba, Japan, reports on a material made almost entirely from carbon nanotubes that retains its viscoelasticity from -196 °C (-321 °F, which happens to be liquid nitrogen temperature) to 1000 °C (1832 °F). That's an astounding temperature range to maintain viscoelasticity compared to most materials we know.

Carbon nanotubes are rolls of graphene, and the Japanese group used a mixture of single-walled, double-walled and triple walled nanotubes. The nanotubes play a role analogous to the long-chain polymers, sliding past each other to rearrange; however, the authors suggest that the carbon nanotube motion is very different than the motion of the molecules in a polymer. The carbon nanotubes are sort of "zipping and unzipping" against each other as they come into contact - that's NOT a thermally activated process and may explain the very wide temperature range over which viscoelasticity is observed. To make things even better, the nanotubes may have some additional elasticity because the tubes can flatten and then re-round themselves.

You know there's a catch, right? Here it is: this only works in a "non-oxidizing atmosphere", meaning the experiments weren't done out in the open where air could get involved. When you heat small-diameter nanotubes to about 400°C in air, they burn. The material would also (at least right now) be pretty expensive, but since the most imperative applications are probably the most extreme, the price might not be as much of an issue for the space program or the defense industry. I don't see this material replacing gel insoles anytime in the near future, but I have a high-temperature furnace that makes me insane because the high-temperature polymer seals I use to protect my samples from oxidation during annealing leak a whole lot more frequently than I'd like them to. This new carbon nanotube rubber is essentially the equivalent of the cookware that goes from freezer to oven without breaking.

Now, if only there is a story that goes with the discovery of this material that can rival the (OK, possibly apocryphal) story surrounding the Goodyear's discovery of vulcanization...

So, co-blogger Diandra was visiting over the weekend, and we ended up partying with UCLA physicist David Saltzberg -- also known as the "Neutrino Whisperer," not to mention the tech consultant for The Big Bang Theory. (Check out his brand new blog about the science behind each episode: "The Big Blog Theory.") David made latkes in honor of Hanukkah, and we conducted an impromptu experiment with the high-tech wine decanting device he'd just received as a gift. Naturally, this required consuming much wine, because of course we had to have control experiments, too. The end result? No blog post this weekend, although Diandra's got a fantastic one in the works about our wine experiment. She just needs to "do a few more experiments." Wink, wink.

In the meantime, in Diandra's honor (she works with nanoparticles for cancer treatments in her day job, and swears adding "nano" to your grant proposal doubles your chances of funding), check out this fantastic nanoscale version of the world's tiniest snowman, courtesy of the UK's National Physical Laboratory. It's part of their "Educate and Explore" initiative, apparently. Not only is Nano-Frosty pretty darn cute and appealing to the public, but he's helping the NPL scientists fine-tune the cantilevers used in atomic force microscopes, among other practical research benefits.

This isn't the kind of holiday decoration you can make at home. According to their Website, Nano-Frosty measures about 1/5th the width of a human bair, and is made from two tin beads that are typically used to calibrate electron microscopes suffering from astigmatism. (Who knew microscopes could have astigmatism?) More details:.

The eyes and smile were milled using a focused ion beam, and the nose, which is under 1 µm wide (or 0.001 mm), is ion beam deposited platinum.

A nanomanipulation system was used to assemble the parts 'by hand' and platinum deposition was used to weld all elements together. The snowman is mounted on a silicon cantilever from an atomic force microscope whose sharp tip 'feels' surfaces creating topographic surveys at almost atomic scales.

I recall reading an article asserting that women living in the same house or dorm would end up with synchronized menstrual cycles. I'm thinking that women who blog together must have some universal rhythm too, since I started working on a "lead post" a week ago - well before Lee posted hers yesterday. Luckily, lead is a dense enough topic that the only overlap between our posts is the Romans.

My inspiration wasn't a half-nekkid HughJackman (see Lee's post), although that is an inspirational vision. I was trying to find out whether L'Oreal sponsored programming on PBS, as I'm looking for a funding source for a program I'd like to do on cosmetics. What came up on Ask.com was a paper from Nanoletters by a group of researchers from L'Oreal R&D (in France) about nanoparticles of lead sulfide (PbS) used by the ancient Greeks and Romans for dying hair black.

Lead has been a cosmetics component for a very long time. Lead white (lead carbonate or 2PbCO3·Pb(OH)2 + PbCO3) was used for foundation by the Hellenes, which is pretty impressive because it takes a lot of chemical synthesis. Pliny the Elder described how to prepare it from metallic lead and vinegar in a paper in a very early edition of JACS. The Volos museum in Greece has power compacts from the end of the 4th Century B.C. Lead white also used to be common in paints, but (as Lee points out), it poses a risk of lead poisoning. The Romans just used talc and gypsum - as did I the one time my husband and I tried drywalling.

The natural look is in now, thankfully, but it surprised me that the many of the Greco-Roman recipes are still being used. Mix lead oxide (PbO) with slaked lime (Ca (OH)2) and a small amount of water. The resulting paste applied to gray or light-colored hair and, after 24-72 hours, turns the hair black. Grecian formula is lead (II) acetate (Pb(CH3COO)2) and it works the same way as the ancient formula. (Just for Men hair coloring does not use lead acetate - but it doesn't work as gradually as Grecian formula. Or so I'm told.)

The protein alpha-keratin makes up most of your hair (as well as your fingernails). (Beta keratin makes up harder things, like bird beaks, reptile claws and scales). Keratin
is a long coiled molecule that acts like a spring, as shown in yellow in the picture at left. Four keratin springs twist together to form a protofibril. Eleven protofibrils twist together to
form a microfibril, which is the largest structure shown in the diagram at left. Keratins have lots of the sulfur-containing amino acid cysteine and that allows the formation of disulfide bridges
that hold together keratin molecules. Think of a disulfide bridge
as two keratin molecules holding hands. The bridge forms a much
stronger structure than the two individual molecules. Sulfur bridges
do the same thing in vulcanized rubber.

Microfibrils pack
together in long thin bundles called macrofibrils. Macrofibrils
pack together to form long thin cortical cells, and cortical cells
pack together to form a hair. Human hair is about 14%
cysteine and most of it is in the grey area in the figure that
separates macrofibrils. There is a fair amount of sulfur in hair,
which is why burning hair is one of the absolute worst smells in the
world - even when it is someone else's hair. Permanent curling and
straightening products break sulfur bridges, reshape the hair, and
then
reform the bridges so that the hair adapts the new shape.To give you an idea of size, the average hair is about 70,000 nanometers. A macrofibril is roughly 7 nanometers in diameter. L'Oreal has a great animation
showing the composition of the hair. The picture at right (from the L'Oreal animation) shows the macrofibrils (the smallest structures visible in that shot) and the cortical cells that make up a hair.

The little black things you see within each macrofibril is melanin, which is responsible for giving you your hair color. Large star-shaped cells called melanocytes reside at the bottom of the hair follicle and manufacture the melanin, which is incorporated into the hair structure as the hair is formed. Only about 1% of the hair is melanin, so it doesn't take much to give it its color.

There are only two types of melanin. Eumelanin is rice-shaped and comes in brown and black varieties. Phaeomelanin is irregularly shaped and imparts a pink to red hue. Japanese hair contains mostly eumelanin and red hair is rich is phaeomelanin. Black eumelanin is in mostly non-Europeans, while brown eumelanin is in mostly young Europeans. A small
amount of brown eumelanin in the absence of other pigments makes hair blond. A small amount of black
eumelanin without other pigments causes grey hair. With no melanin, hair is white, although we don't know yet whether that is because your body stops producing melanin, or if it just isn't incorporated into the hair.

When the coloring formula interacts with the hair, the lead in the colorant combines with the sulfur in the hair and forms nanoparticles of lead sulfide (PbS) with diameters between 4 nanometers and 15 nanometers. In contrast, natural melanin that produces black hair is about 300 nanometers in diameter. The longer you leave the formula on the hair, the more nanocrystals you form and the blacker the hair looks. This is why coloring products like Grecian formula slowly change the hair color and can get rid of grey gradually, unlike permanent color. The PbS nanocrystals are very small and it doesn't take a lot of them to change the apparent color of the hair, so the mechanical properties of the hair aren't really affected.

Why do they form nanocrystals and not microcrystals? One theory is that peptides - polymers that surround the organized keratin proteins - form nanoreactors that limit the size of the PbS nanoparticles. The nanoparticles accumulate preferentially at the boundaries between the microfibrils, whereas melanin colorants are randomly distributed throughout the hair.

L'Oreal presents the L'Oreal-UNESCO awards each year to outstanding women scientists across the world: one in African/Arab countries, one in Europe, one in North America, one in Asia, and one in Latin America. The awards recognize the important role science plays in their industry and in the rest of the world. They also offer a variety of awards and fellowships for women at other stages in their careers.. because we're worth it.

Fans of Douglas Adams' Hitchhiker's Guide to the Galaxy are familiar with the fictional Infinite Improbability Drive that powers the spaceship Heart of Gold. It allows for faster-than-light travel, per Adams, and is based on one of the central peculiarities of quantum physics: the notion that a subatomic particle exists in a superposition of states until it is observed and its wave function collapses into a definite state. Until then, every possible state -- however improbable -- exists simultaneously. As applied to the Infinite Improbability Drive, this means that as the drive reaches infinite improbability, the ship will pass through every conceivable (and inconceivable) point in every conceivable (and inconceivable) universe; the ship is literally everywhere at once, and you can then decide at which point you want to be when the improbability levels return to normal. Ergo, a body can travel from one place to another without passing through the intervening space -- provided you have sufficient control of probability.

This is easier said than done, of course: an earlier deployment of the improbability drive on board the Starship Titanic was designed to make it infinitely improbable that anything could go wrong. Instead, the deployment supposedly ended in a "Spontaneous Massive Existence Failure," presumably because it was not fully appreciated that "any event that is infinitely improbable will, by definition, occur almost immediately." Then there's the fact that human beings can find travel by improbability a distressingly surreal experience: they can turn into sofas, lose limbs, nuclear missiles can morph into sperm whales, and in this clip from the film version of Adams' novel, the Heart of Gold morphs into a giant ball of yarn.

But the savvy sci-fi enthusiast also knows that the drive is based on Brownian motion: the random jittery movement of particles suspended in a liquid or gas (a nice hot cuppa tea in the case of the Infinite Improbability Drive), which in turn gave rise to a mathematical model for describing such random movements that has found any number of real-world applications (although not, to date, in an Infinite Improbability Drive). Back around 60 BC, the Roman poet Lucretius penned this description of the random motion of dust particles, which he used as proof of the existence of atoms (a controversial view at the time):

"Observe what happens when sunbeams are admitted into a building and shed light on its shadowy places. You will see a multitude of tiny particles mingling in a multitude of ways.... their dancing is an actual indication of underlying movements of matter that are hidden from our sight.... It originates with the atoms which move of themselves. Then those small compound bodies that are least removed from the impetus of the atoms are set in motion by the impact of their invisible blows and in turn cannon against slightly larger bodies. So the movement mounts up from the atoms and gradually emerges to the level of our senses, so that those bodies are in motion that we see in sunbeams, moved by blows that remain invisible."

Lucretius was on the right track with his observations, all those centuries ago, although he didn't account for the effect of air currents on the "mingling motion" of the dust motes. Nobody really commented significantly on the phenomenon again until 1785, when Jan Ingenhousz discussed the strange motion of coal dust particles on the surface of alcohol. But he isn't credited with the "discovery" of Brownian motion, which is probably a good thing, since "Ingenhouszian motion" doesn't have quite the same ring to it.

The name derives from the 19th century botanist Robert Brown, who was studying pollen particles floating in water under the microscope. Within those grains of pollen, he noticed even smaller particles jiggling in seemingly random motions. Augh! They were alive! Well, not quite. Brown was a scientist, refused to panic, and repeated the experiment with particles of dust. He saw the same kind of thing, and thus concluded that the motion did not occur because the pollen particles were "alive." (Brown's original paper is here.) Today, of course, scientists understand the underlying mechanics of Brownian motion, and appreciate its importance as a means of indirectly confirming the existence of atoms and molecules.

Say you've got a grain of pollen moving about randomly in a bowl of water. The pollen is a good 250,000 times larger than the water molecules that make up the water in the bowl. With the naked eye -- or even a simple microscope, like the one Brown used -- we can only see the pollen, which seems to move randomly of its own accord. What we can't see are the much smaller water molecules, which are jiggling in their own form of thermal motion. Those smaller water molecules collide with the pollen grain constantly, from all different directions, which should average out to little or no movement. But there are always tiny imbalances at any given time: say, 20 water molecules exerting a force pushing the pollen to the right, and maybe 22 water molecules "pushing" to the left. Because of this slight imbalance, the pollen will move ever-so-slightly to the left. And that's why we get random Brownian motion with grains of pollen suspended in water.

I found myself musing on Brownian motion while listening to a talk by Harvard University biophysicist Adam Cohen during the 2008 Industrial Physics Forum in Boston. See, the smaller an object is, the faster it will jiggle, and since individual atoms and molecules are very small indeed, this constant motion really interferes with high-resolution imaging. How do you pin down a single molecule long enough to really prove its physical properties in depth? Sure, scientists now routinely use laser tweezers to trap and cool atoms, and it's a powerful tool, indeed. But the smaller the sample the more power is needed to hold a molecule in the trap, and at some point so much power is needed that it "cooks" the molecule instead of just trapping it.

Cohen -- a relatively spanking new PhD who looks like a teenager (or maybe I'm just getting old) -- did his thesis work on coming up with a viable solution: the Anti-Brownian ELectrokinetic trap (or ABEL trap), which can pin down single molecules at room temperature. It's an ingenious combination of many different scientific tools developed over the last 15-20 years. You need a fluorescently labeled molecule of interest -- a polystyrene nanosphere, for instance, or maybe a bit of tobacco mosaic virus -- a fluorescent microscope to track the molecule, and a smidgen of laser light on the order of mere microwatts.

The basic idea is to slow down the molecule's instantaneous motion by zapping it with carefully timed bits of electricity, via electrodes surrounding the sample -- albeit at a safe enough distance to ensure no unwanted chemical effects are produced. Oh, and did I mention the microfluidics? The little "kicks" of electricity get transmitted to the molecule via tiny micro-channels in an underlying chip. Cohen also added glycerol to the solution to increase the viscosity a bit more and further slow down the Brownian motion.

It's essentially a real-time electrokinetic feedback process that can be used to control the motion of individual molecules: basically, those carefully timed electric jolts induces an electrokinetic "drift" that cancels the natural Brownian motion of the molecule. (Per Cohen, the feedback mechanism plays the same role as "Maxwell's Demon," enabling the system to seemingly "violate" the second law of thermodynamics by wringing order out of randomness.) The faster this process can be applied, the more efficient the trapping mechanism will be. So far, Cohen has used the ABEL trap to study fluorescent quantum dots, DNA molecules, fluorescently labeled lipid vesicles, single particles of the tobacco mosaic virus (the image above shows the actual trajectories of 13 such TMV particles held in an ABEL trap), and single molecules of large complex proteins, as well as fluorescent polystyrene nanospheres and cadmium-selenium nanocrystals.

Generation 1 of the ABEL trap employed microfluidics in a kind of "Magic Wand Device" for the feedback, said Cohen: four photoresistors in a diamond pattern embedded in a wand, which had the added advantage of being incredibly cheap (50 cents each). He could then drag the wand across a monitor to move the particles, and it worked as long as no shadows fell across the screen. The brilliance of the microfluidic cell is that ABEL can move both charged and neutral particles -- the latter via a sort of "electro-osmotic" effect due to hydrodynamics forces (i.e., the particles literally "go with the flow"). For Generation 2, Cohen developed a software-based feedback mechanism to track the tagged single molecules, combined with CCD imaging. Now he can click on an image of the particle on a computer screen to move it around. And ABEL still allows scientists to study the dynamics of single molecules, since "the center of mass is immobilized by the feedback, but the internal dynamics are unchanged," says Cohen.

But the best part? Cohen's control over the movement of the particles using this real-time electro-kinetic feedback is so precise, he even managed to make a movie showing particles in an ABEL trap "subject to an arbitrary waveform": i.e., the eminently danceable "I Like to Move It, Move It" song from the animated film Madagascar. You can see Cohen's (very short) movie here -- make sure your sound is on! -- and below, for your aural edification, the "music video" from Madagascar's credit sequence, featuring the entire "cast," including Julian the Groovy King of the Lemurs (voiced by Sacha Baron Cohen). Get down with your bad selves, y'all!

DyingIs an art, like everything elseI do it exceptionally wellI do it so it feels like hellI do it so it feels realI guess you could say I've a call -- "Lady Lazarus," Sylvia Plath

The poet Sylvia Plath has always held a fascination for me, not because of her repeated suicide attempts (she succeeded on the third) and obviously troubled nature, but because of the stiletto-sharp clarity of her poetry, and how much raw emotion she managed to convey in such tightly minimalist phrasing. "Lady Lazarus" is one of the most famous poems in her final collection, Ariel, brimming over with barely controlled white-hot rage. Scholars have analyzed it endlessly for autobiographical references to Plath's life (her daddy issues alone would fill tomes) and self-induced brushes with death, but what comes through for me in the poem is Plath's thinly-veiled contempt for those who were drawn to her precisely because she kept chasing after, and miraculously cheating, death.

The imagery presents the poem's persona as the featured act in a carnival sideshow, a person craving attention who is as addicted to the applause of the audience when she survives as they are fascinated by her death-defying (or seeking) feats. Plath tapped into a disturbing facet of our celebrity-obsessed culture: the glorification of nihilistic behavior, particularly of the "live fast, die young, and leave a good-looking corpse variety." There's something almost cannibalistic about our fascination with tragic figures in popular culture: we can't get enough news about their personal weaknesses and tragedies. Nowhere is this more evident than in the music industry, which is littered with the corpses of talented musicians who bought into this empty philosophy and paid dearly for it: Jimi Hendrix, Janis Joplin and Kurt Cobain spring immediately to mind, but they're in very good company.

The most recent train wreck in the music industry is pop singer Amy Winehouse, whose LP Back in Black showcases an extraordinary talent -- and whose personal life is a shambles, to say the least. These days, she looks more like a strung-out homeless junkie than one of the biggest stars in the world. The drinking, the smoking (both cigarettes and crack), the ecstasy, the ketamine, have taken their toll: at age 24 Winehouse shows early signs of emphysema and an irregular heartbeat. On July 28, she was rushed to the hospital for an "adverse reaction to medication" -- a claim that was greeted with more than a little skepticism, given her public touting of her drug use.

Sure, it's a waste of talent and human life, but it's her choice, so why even bring it up? You might be thinking. But I can't help feeling twinges of compassion for Winehouse, who has bought into that false philosophy, just as I have compassion for anyone who has despaired so completely that death seems a reasonable option to their current life. Clearly, a new role model is needed to show us how to live, and how to die -- not with a nihilistic shaking of the fist in defiance of god-knows-what imagined enemy, but with grace, good humor and dignity. We have that role model in former Carnegie-Mellon University computer scientist Randy Pausch, who died of metastasized pancreatic cancer two days before Winehouse was rushed to the hospital for her "adverse reaction to medication."

Pausch needs no introduction, really: his famous "last lecture" at Carnegie Mellon has been downloaded and viewed by millions all over the world, and his passing was marked by every major media news outlet, and all throughout the blogosphere. If you are one of the three people who haven't seen the lecture yet, it's an hour or so well spent.

Pausch was breathtakingly candid and accepting of his fate, joked about being able to do lots of pushups (which he ably demonstrated), and wisely refused to talk about his wife and children, who would be most affected by his death. Mostly, he talked about life: how to live well by pursuing your childhood dreams, not getting discouraged by the brick walls (they're there to test how much you want something), and how to leave some sort of personal legacy behind... even if it's just for your wife and kids. His is a memorable example of a life well-lived, and in choosing to share his lecture (and his fate) with the world, he also showed us how to die. As Wall Street Journal columnist Jeffrey Zaslow memorably observed, "His fate is ours, sped up."

I simply can't watch that lecture without getting all choked up, and I couldn't bring myself to blog about Pausch's passing until now. He gave us something very precious, you see, and we couldn't return the favor by saving his life. The cancer was inoperable; as it is, he lived five months longer than doctors expected. As Pausch knew better than anybody, there is no magical deus ex machina enabling us to cheat our common fate. But that doesn't mean scientists aren't trying. The day after he died, the American Association of Physicists in Medicine (AAPM) kicked off their annual meeting in Houston, Texas.

Just this past March, a very ill Pausch summoned the strength to testify before a House committee, requesting more funding for pancreatic cancer research. So the only way I can think to honor him is by highlighting some of the research presented at the AAPM meeting. Currently celebrating its 50th anniversary, the AAPM is dedicated to advancing the application of physics to the diagnosis and treatment of human disease -- including cancer. As Pausch's family were making funeral arrangements, speakers at the AAPM meeting were describing the latest batch of innovative R&D that may one day make cancer a thing of the past.

One of the major obstacles to removing tumors via surgery is that not all of them have very well-defined borders, making it difficult to remove the tumors entirely without leaving a few errant cancerous cells behind -- which continue to grow and multiply wildly until more tumors appear. There's also greater risk of complications, with long recovery periods. Researchers have been experimenting for years with various nanoparticle-based therapies. The basic concept is this: tailor the nanoparticles to gravitate towards cancerous tumors and lodge inside them, then deliver laser heat to that area; the nanoparticles will burn away the tumor and leave the healthy cells behind.

The latest twist on this sort of thermal ablation therapy comes from a group of researchers at the University of Texas' cancer center, who are also working on better ways to precisely guide and concentrate laser-generated heat in targeted tumors. It would help if we could see the process while it happens. In this case, they injected gold silica nanoshells into brain cancer models. As expected, the nanoshells made a beeline for the target tumors and were handily taken in like long-lost friends. Then the UT scientists applied low-power laser light to selectively heat and burn away the tumor but not the surrounding healthy tissue. The twist: they added iron-oxide cores to the nanoshells, thereby making it possible to visualize them using magnetic resonance imaging (MRI). Their conclusion? "[T]he use of magnetic resonance temperature imaging and gold nanoshells hold the very real possibility of meeting the long-sought goal of improving the precision of thermal ablation, while sparing healthy tissue."

Since not all parts of a tumor will respond the same to conventional radiation therapy, which uses a uniform dose to the entire tumor. In the future, it might be more effective to target the more resistant parts of a tumor with stronger doses. It's known as "dose painting" in medical physics circles -- or, more technically, as intensity modulated radiation therapy (IMRT). This means we'd need to be able to measure the degree of resistance in those different parts accurately -- right down to the molecular level -- and it turns out that this is no small feat, according to a new analysis by scientists at the Institut Jozef Stefan in Ljubljana, Slovenia, and the University of Wisconsin, Madison.

The conventional method relies on PET scans, which can be quite useful in measuring radio-resistance if the radio tracer fluoro-L-thymidine (FLT) is used. Cells grab onto FLT as they divide, and since cancer cells divide so rapidly, they pick up more of the FLT, and hence look brighter on a PET scan.(It's called a standardized uptake value.) If those cells in that region are still bright after a round of radiation therapy, it's pretty clear they're radio-resistant, and the therapy is ineffective on those cells.

IJS's Urban Simonic thinks there is another, more precise way to find the radio-resistant cancer cells by using a more dynamic approach: modeling how the radio-tracer travels through the body and is collected by cells over time. He and his Wisconsin colleagues compared the two techniques using the same set of PET scans, and found that the two approaches selected different regions as being resistant to radiation therapy. Oops. Whether or not one agrees that Simonic's approach is more precise or not, the discrepancy clearly needs to be addressed if dose painting is ever going to clinically effective.

Researchers haven't been idle on the dose painting (or IMRT) front either. Another paper at the AAPM meeting dealt with a new variant called volumetric modulated arc therapy (VMAT). Developed by researchers at the Memorial Sloan-Kettering Cancer Center, the method offers the same treatment as IMRT in roughly half the time. That's because, with IMRT, a computer-controlled linear accelerator sweeps a narrow slit of radiation across the tumor from various angles around the patient, one angle at a time. The VMAT technique -- at least the variant described at the meeting -- breaks that arc into 360 evenly divided beams. The researchers developed a computer program that adjusts the aperture shape and radiation does for each one of those 360 beams. And because the resulting aperture beams are much larger than in IMRT, treatment time is reduced significantly (up to 50%), along with the patient's exposure to any radiation leakage (down from 5 minutes to 2-1/2 minutes).

That's literally just the tiniest smattering of the fascinating research being done in this area, by PhD physicists, doctors, pharmacologists, oncologists, neurosurgeons, and so forth crossing disciplinary barriers to fight a common foe. None of this new cutting-edge technology came of age in time to save Pausch and the the millions of other people around the world who die from some form of cancer each year -- all of whom have friends and family and colleagues devastated by their loss, even they don't warrant front-page obituaries in all the major newspapers. But we can honor them by continuing to fight the onslaught of disease, and we when we lose -- as indeed, we must -- by dying exceptionally well.

Celebrity tabloids and similar gossip rags are filled with unnamed sources -- you know, "Sources close to [Insert favorite Celebrity (TM) here] report that...." I've always wondered who these loose-lipped people are: a source standing close to the Celebrity (TM) on line at Starbucks, perhaps? A casual diner at the next table in a stylin' Hollywood eatery? Or maybe a disgruntled former employee, or pathetic former hanger-on who's bitter because s/he didn't become the Celebrity's (TM) new BFF? Because anyone who was truly close to a Celebrity (TM), wouldn't maintain that position for very long by talking to unscrupulous tabloid reporters, now, would they?

Tabloids aside, we generally expect the average news story to cite its sources by name, with rare exceptions, like when Woodward and Bernstein broke the Watergate scandal. So imagine my surprise when I noticed this little news item on CNET a couple of days ago about a start-up company called Stion with the stated mission of developing thin-film solar cells, mentioning unnamed "sources" in the third graph down. They've received a whopping $15 million in venture capital, which means their technological approach must be pretty promising, but Stion's manager of business development was rather coy about what, exactly, that material might be. He had plenty to say about what it was not -- not silicon based, not cadmium telluride, and not a copper-indium-gallium-selenide compound, either -- but otherwise kept mum, promising to reveal all "in due time," i.e., around 2010, when the first products are likely to be announced.

Sheesh, what a media tease! Clearly, the frustrated CNET reporter, Michael Kanelios, had no choice save to turn to his own Deep Throat, or two. Which, BTW, is not a criticism: he did a commendable job (far better than the celebrity tabloids) of assembling various pieces of circumstantial evidence in favor of the new material being... (drum roll, please)... quantum dots! I'm not sure his evidence is solid enough to warrant announcing this in the headline ("Harnessing quantum dots for solar panels") without a question mark, unless his unnamed sources are very reliable indeed. But it is a matter of public record that Stion's Chief Technology Officer, Howard Lee, worked as a solar researcher (specifically using quantum dots) at Lawrence Livermore National Lab for several years, and accumulated numerous patents relating to quantum dots during his stint at another start-up called Ultradot. And Stion's CEO, Chet Farris, is a former president of Shell Solar. Hmmm. The plot thickens. Kanelios, for one, can connect the dots.

No doubt some of you are wondering what the heck these quantum dot thingies are, and why they're such a big fat deal. It just so happens that I wrote about quantum dots way back in 2003 for The Industrial Physicist magazine -- the closing of which I still mourn, because I got to cover so many cool cutting-edge topics as a contributing editor there. (I wasn't always about the pop culture physics; I used to work for the Dark Side, i.e., Very Serious Science Journalism, or VSSJ.) Quantum dots are essentially tiny bits of semiconductors -- sometimes called nanocrystals, which just doesn't carry the same panache -- just a few nanometers in diameter. It's like taking a wafer of silicon and cutting it in half over and over again (a semiconducting Zeno's Paradox?) until you have just one tiny piece with about a hundred to a thousand atoms. That's a quantum dot. (I think. It's not like you can actually see one with the naked eye. Billions of them could fit on the head of a pin.)

Size matters when it comes to semiconductors: smaller is usually better. Because they're so tiny, quantum dots have some unusual materials properties -- specifically, the all-important electrical and optical ones -- thanks to the quantum effects that kick in at smaller size scales, so they are of enormous interest to researchers. It's interesting physics fundamentally, and it offers an impressive sampling of potentially lucrative practical applications. Trust me, quantum dots are hot, even if they're currently simmering on the back burner in the news-hook-oriented media.

I must confess to finding it easier to write about applications of physics rather than the basic science. But when I started covering the quantum dots area, I learned some useful things about the "electrons and holes" effect that is critical not just to quantum dots, but also lasers and other semiconductor physics. This is not an easy thing for a lay person to visualize, although physicists toss those terms around like high school slang. So here's my attempt at the 411 on electrons and holes (scientific commenters, feel free to add your own take on the subject):

It helps to place semiconductors in general in the appropriate context, i.e., right smack between insulators and conductors. Insulator atoms hoard their electrons greedily, like misers or overprotective parents, and rarely part with them, while conductor atoms are like spendthrifts or exceedingly permissive parents, letting their electrons run amok all over the place (and a good thing, too, otherwise we'd never enjoy the benefits of electrical current). Semiconductor atoms are juuuust riiiight. They don't fling their electrons around all willy-nilly, but neither do they hang onto to them too tightly. It takes a bit of an energy boost to knock an electron loose in a semiconductor, and when the electron breaks free, it leaves behind a "hole" in the atom's electronic structure -- a vacancy, if you will, that another electron, sooner or later, will come along to fill. So a photon strikes a semiconductor atom and creates an electron-hole pair, which physicists call an exciton -- because we need more confusing technical jargon in physics, don't we? Anyway, this enables the electrons to flow as a current. And current = power.

Much of the excitement over quantum dots stems from a decades-long quest to make silicon emit light efficiently in the visible spectrum. Back in 1990, European researchers managed to get porous silicon to emit red light, and figured it came about because of "quantum confinement" relating to the dot's small size. Basically, at 10 nanometers or less, the electrons and holes are being squeezed into such small dimensions that this alters the electronic and optical properties; it's the critical feature of most nanoscale materials, frankly. (Special bonus for Physics-Philes: a 2003 paper in Nature reported that shape might matter as much as size when it comes to quantum confinement.) Things snowballed from there, with scientists making more silicon dots (and, later, germanium dots) that emitted light in lots of bright, pretty colors, especially the highly desirable green and blue ranges. Basically, the bigger the dot, the redder the light, and the emitted light becomes shorter and shorter in wavelength -- and higher in energy -- as the dots shrink in size. This is called "tunability" because you can pretty much tailor the dots to emit whatever frequency of visible light you happen to need for a given application, simply by altering the size of the dots. Believe me, high-tech industries go nuts for anything with tunability. Plus, colors = pretty! Check out the pic! Doesn't it make you want to buy some quantum dots?

The most obvious application is using quantum dots as an alternative to the organic dyes used to tag reactive agents in fluorescence-based biosensors. You know, the dyes start to glow when, say, a harmful toxin is present. But the number of colors available using organic dyes is limited, and they tend to degrade rapidly. Quantum dots offer a broader spectrum of colors and show very little degradation over time. Having all those colors also means you can make light-emitting diodes (LEDs) from quantum dots, precisely tuned in the blue or green range. You can also build quantum dot LEDs that emit white light for laptop computers or interior lighting in cars. As for electronics, the possibilities are endless: all-optical switches and logic gates, for instance, with a millionfold increase in speed and lower power requirements, or, further in the future, quantum dots could be used to make teensy transistors for nanoelectronics.

But the current news is about quantum dots and their potential application to solar cells. As I mentioned earlier, Stion's CTO, Lee, did a lot of work in this area during his stint at Livermore, as have many other researchers. (A more technical overview of this research area by Science News' Peter Weiss can be found here, for those who are interested.) As the CNET story reports, "Most solar cells on the market today extract electricity from sunlight with silicon and are integrated into glass substrates, which is relatively heavy." A company called First Solar uses a similar structure, but replaces the silicon with cadmium telluride, which is cheaper. As for the CIGS (copper, indium, gallium and selenide) version of the technology, there's several companies working on that, although products have yet to hit the market. It's expected that CIGS solar cells will be cheaper than silicon ones, but not quite as efficient: we're talking mid- to low teens, percentage wise, compared to 22% efficiency (and sometimes as much as 29%) for silicon solar cells. (A fossil fuel like gasoline can show 30-40% efficiency, so even silicon solar cells need to show some improvement in this area for broad practical application.)

So what can quantum dots bring to the solar cell table? The CNET article doesn't go into much detail:

"Partly because of their small size, quantum dots can be highly sensitive to physical phenomena and can be used to trap electrons. Since solar panels work by wiggling electrons out of sunlight and transferring them to a wire, quantum dots in theory could work well in solar panels."

This doesn't really say anything meaningful, to a non-scientist, about the actual process that's taking place -- because the process is very complicated and hard to explain in a short news article. Those electron-hole pairs known as excitons I mentioned earlier? Usually, photons from sunlight that strike the semiconductor material used in solar cells unleash only one electron. In theory, they should be able to loosen more than one, thereby giving rise to several excitons, but for reasons relating to heat loss in atomic collisions, or some such thing (paging Chad Orzel for a better explanation!), there's usually only a 1-to1 ratio. That's why solar cells are limited in their energy efficiency. But last year, researchers working with quantum dots made of lead selenide found they could produce as many as seven excitons (electron-hole pairs) from a single high-energy photon of sunlight. This could boost solar cell efficiencies to as much as 42% -- enough to be competitive with the more common fossil fuel energy sources.

So Stion is doing good work, and with any luck, they'll be rewarded with some healthy profit margins in due time. But all this hard-core quantum physics talk has made me long for the comparable simplicity of the celebrity tabloids. Intellectual stimulation is all very well and good, but sometimes my brain just needs a break. And the latest unscrupulous unnamed sources are telling me there's trouble in some Celebrity's (TM) fairytale paradise...

One of the best-known poems by Gerard Manley Hopkins -- a Victorian-era minister whose writings frequently centered on the glories he observed around him in nature -- opens with a tribute to the phenomenon of iridescence: the wings of kingfishers and dragonflies, in Hopkins' poem, but it can also be found in the wings of cicadas, and butterflies, in certain species of beetle, and in the brightly colored feathers of male peacocks. A firm believer in divine hierarchy, Hopkins found a metaphor for man's relation to God in this peculiar attribute of nature: "Each mortal thing does one thing and the same/... Crying 'What I do is me, for that I came.'"

I don't share Hopkins' religious ecstasy, but I have always appreciated his skillful use of language and meter, and his unabashed appreciation for the natural world. Nature fits form to function, and everything has its place in the delicately balanced ecosystem. You don't need to believe in God to marvel at that, or at the many examples of iridescence in the world around us. Equally marvelous is the unusual cause of those bright flashes of hue. The color we see doesn't come from actual pigment molecules, but from the precise lattice-like structure of the wings (or shells, or feathers), which forces light waves passing through to interfere with itself, so it can propagate only in certain directions and at certain frequencies. And the brilliant colors that result change depending on one's point of view. In essence, they act like naturally occurring diffraction gratings.

Physicists call these structures photonic crystals, an example of "photonic band gap materials", meaning they block out certain frequencies of light and let through others. (If you prefer an explication of the science from America's beleaguered pop princess, Jen-Luc Piquant suggests you check out Britney Spears' Guide to Semiconductor Physics. Now that Britney has lost the baby weight and the loser husband, Jen-Luc fervently hopes she will return to her cutting-edge physics research.) This makes them "tunable", particularly the manmade varieties, because of those highly ordered arrays of periodic "holes". Anything tunable is by definition controllable, and therefore useful for practical applications. Photonic crystals are used most often as waveguides for light in telecommunications/fiber optics systems, or other places where scientists want to be able to control either the frequency or the direction of light.

Over the last six months, there's been several interesting new developments in the effort to exploit the features of naturally-occurring photonic crystals in innovative ways. (I've been collecting newsy items on the topic for several months now, in hopes of finally finding time to write a blog post about it. That time has come.) Most recently, in November, Chinese researchers (Jin Zhang and Zhongfan Liu) at Peking University announced that they have figured out a way to use the wings of cicadas as stamps to pattern polymer films at nanometer size scales -- a feat that is quite challenging using conventional microfabrication technology. The wings are rigid enough so that when they are pushed down onto a smooth polymer film, that film is imprinted with a negative version of the array pattern.

The wings are also chemically stable, plus they have a waxy coating which results in very low levels of surface tension. This is important, because the wings don't end up getting stuck to the polymer film after imprinting. They can be removed while leaving the stamped pattern intact. That pattern is then transferred to silicon via a more traditional etching process, thereby forming "nanowells" on a silicon chip. Such chips "show promising anti-reflective properties," according to Zhang and Liu, and could be useful for optical imaging, or in the use of Raman spectroscopy for detecting molecules. Liu phrased it best: "There is a lot that nature can teach us about nanotechnology."

Butterfly wings get their color from naturally occurring photonic crystal structures in scales made of chitin, a polysaccharide that shows up in all kinds of insects. Those scales are arranged like tiles on a roof, except they measure a mere tens of micrometers across. Last September, New Scientistreported that a group of researchers have measured the structure and optical characteristics of the photonic crystals in butterfly wings for the very first time. They did it by studying electron microscope images of the scales. It turns out that each side of the wing contains different photonic structures: a metallic blue produced by single crystals, and a dull-ish green that results from a more random arrangement of crystals. Precisely ordering the lattice structure is critical to achieving the most brilliant colorful effects -- and to controlling the propagation of light at the desired frequencies. Which is why telecommunications applications rely on manmade photonic crystals rather than nature's more random arrangements.

Still, butterfly wing crystals can produce green, yellow and blue colors, depending on their overall effect, and the researchers managed to generate red reflections as well. That's significant because such a palette could be used in flat panel displays, simply by mounting an array of crystals only tiny MEMS arms to change their orientation. So any given "pixel" could produce red, green or blue. A September 1, 2006 paper in Optics Letters by a team of Swiss scientists described a similar approach using diffraction gratings and piezoelectric polymers (which contract whenever an electric voltage is applied) to faithfully reproduce a fuller range of colors than can currently be achieved in conventional displays, whether they be standard TVs, LCDs, or plasma screens. (For instance, they can't reproduce the blues observable in the sky or in the sea.) Manuel Aschwanden of the Swiss Federal Institute of Technology in Zurich headed the project, and described the grating as having one side molded into something that looks for all the world like microscopic pleated window shades.

More frivolously, copying the structure of butterfly wings is giving rise to spiffy new kinds of make-up, giving a whole new meaning to the term "butterfly effect." For instance, L'Oreal offers eye shadow, lipstick and nail polish featuring these iridescent effects, bringing nature's beauty to the cosmetics counter. This is achieved by stacking nanoscale layers of materials like mica, silica or liquid crystals, of varying thickness to give each material a specific refractive index. For instance, a stack 80 nanometers high produces blue, while one 120 nanometers high produces red. In the package, though, the stuff just looks white; the colors appear when the makeup is applied and exposed to light. There are the usual concerns about using nanoparticles in cosmetics, when little is known to date about potential health risks, but that hasn't dampened the enthusiasm for such novelties. Yet.

Researchers at the University of Toronto have developed a new elastic type of photonic crystal that changes color with the application of pressure. It also mimics the structure of butterfly wings and opal (the gemstone is another common example of a naturally occurring photonic crystal): it resembles a 3D honeycomb. They hope to develop the material further in hopes of using it to, for example, capture full-color fingerprints. The obvious advantage is the enhanced contrast and sensitivity to detail, making it easier to analyze prints for identification purposes. But any impression picked up by the elastic photonic crystal is visible immediately in bright hues, with no need to first convert that raw data into electrical signals for computer analysis. The Toronto material could also be used as pressure sensors in consumer electronics or airbag deployment -- or just for children's toys.

Imagine a toddler being able to squeeze a toy and watch the color change right in front of his/her eyes! Imagine the wonder the child would feel, especially when s/he was old enough to realize that it wasn't magic, but a one that arises from Nature itself, that man has seen fit to copy and put to good use. I think even Hopkins would be suitably impressed at what the scientific study of a simple butterfly's wing has wrought. So it seems fitting to close by quoting another Hopkins' poems, "God's Grandeur." It presents a vivid image of the Holy Ghost brooding over our imperfect world "with warm breast and with ah! bright wings."

Physics Cocktails

Heavy G

The perfect pick-me-up when gravity gets you down.
2 oz Tequila
2 oz Triple sec
2 oz Rose's sweetened lime juice
7-Up or Sprite
Mix tequila, triple sec and lime juice in a shaker and pour into a margarita glass. (Salted rim and ice are optional.) Top off with 7-Up/Sprite and let the weight of the world lift off your shoulders.

Any mad scientist will tell you that flames make drinking more fun. What good is science if no one gets hurt?
1 oz Midori melon liqueur
1-1/2 oz sour mix
1 splash soda water
151 proof rum
Mix melon liqueur, sour mix and soda water with ice in shaker. Shake and strain into martini glass. Top with rum and ignite. Try to take over the world.